Lurking behind the Supreme Court’s ruling last week that the Second Amendment protects an individual right to bear arms were a series of fascinating, disputed and now in many ways irrelevant questions. Do gun control laws reduce crime? Do they save lives? Is it possible they even cost lives? Justice Stephen G. Breyer, one of the dissenters in the 5-to-4 decision, surveyed a quite substantial body of empirical research on whether gun control laws do any good. Then he wrote: “The upshot is a set of studies and counterstudies that, at most, could leave a judge uncertain about the proper policy conclusion.”

There is no question, of course, that guns figure in countless murders, suicides and accidental deaths. Over the five years ending in 1997, the Justice Department says, there was an average of 36,000 firearms-related deaths a year. (Fifty-one percent were suicides, and 44 percent homicides.) Determining whether particular gun control laws would have, on balance, prevented some of those deaths is difficult. Take Washington, D.C., whose near-total ban on handguns in the home was on the receiving end of last week’s decision.

At the crudest level, as Justice Breyer wrote, violent crime in Washington has increased since the ban took effect in 1976. “Indeed,” he continued, “a comparison with 49 other major cities reveals that the district’s homicide rate is actually substantially higher relative to these other cities than it was before the handgun restriction went into place.”

Those statistics by themselves prove nothing, of course. Factors aside from the gun ban, like demographics, economics and the drug trade, were almost certainly in play. “As students of elementary logic know,” Justice Breyer wrote, “after it does not mean because of it.”

But Gary Kleck, a professor at Florida State University’s College of Criminology and Criminal Justice, whose work Justice Breyer cited, said there were good reasons for making a definitive judgment.